Lathe cutting capability is limited by the dynamic stiffness of the machine tool structure. Cutting tools supported by narrow rams at long extensions on vertical lathes provide the reach to cut tall parts, but often lack adequate stiffness. The value of dynamic stiffness varies with frequency and is lowest at specific frequencies related to structural natural frequencies or modes. The minimum stiffness can be 50% or less than the stiffness at the rest of the frequencies, and defines the cutting capacity of the machine. The frequency, stiffness, modal mass, and damping can vary as the machine elements are moved over their operating ranges.
The critical characteristics of dynamic stiffness are static stiffness, mass, and damping. Static stiffness and mass are often dictated by the machine type and size and can be difficult to alter significantly. Damping which may be defined as the ability to absorb or neutralize vibrating energy can be increased through the addition of machine elements. Greater damping results in higher stiffness.
Adding a damper to the ram near the cutting tool will increase the cutting capacity of a vertical lathe. A spring-mass-damper may be tuned to a particular frequency for operation of the ram at the greatest extension, the lowest stiffness condition for the ram.